Heretofore it has been known that electron emission could be obtained from a porous billet as for example a porous tungsten billet that had been impregnated with a barium containing compound such as Ba.sub.3 Al.sub.2 O.sub.6. The Ba.sub.3 Al.sub.2 O.sub.6 impregnant reacts with the wall of the porous tungsten billet generating free barium. The free barium then migrates to the surface by Knudsen flow to give electron emission.
The difficulty with this concept is that there is no mechanism for the generation of electrons. The concept of barium migrating to the surface to give off electrons is too generalized in that if 15-20 mg of barium containing compound gave free barium that was responsible for electron emission, the cathode would cease to give emission within minutes.